There has been a growing interest in the manufacture and use of microfluidic systems for the acquisition of chemical and biological information. In particular, when conducted in microfluidic volumes, complicated biochemical reactions may be carried out using very small volumes of liquid. Among other benefits, microfluidic systems increase the response time of reactions, minimize sample volume, and lower reagent consumption. When volatile or hazardous materials are used or generated, performing reactions in microfluidic volumes also enhances safety and reduces disposal quantities.
Traditionally, microfluidic devices have been constructed in a planar fashion using techniques that are borrowed from the silicon fabrication industry. Representative systems are described, for example, in some early work by Manz et al. (Trends in Anal. Chem. (1990) 10(5): 144-149; Advances in Chromatography (1993) 33: 1-66). In these publications, microfluidic devices are constructed by using photolithography to define channels on silicon or glass substrates and etching techniques to remove material from the substrate to form the channels. A cover plate is bonded to the top of the device to provide closure. Miniature pumps and valves can also be constructed to be integral (e.g., within) such devices. Alternatively, separate or off-line pumping mechanisms are contemplated.
More recently, a number of methods have been developed that allow microfluidic devices to be constructed from plastic, silicone or other polymeric materials. In one such method, a negative mold is first constructed, and plastic or silicone is then poured into or over the mold. The mold can be constructed using a silicon wafer (see, e.g., Duffy et al., Analytical Chemistry (1998) 70: 4974-4984; McCormick et.al., Analytical Chemistry (1997) 69: 2626-2630), or by building a traditional injection molding cavity for plastic devices. Some molding facilities have developed techniques to construct extremely small molds. Components constructed using a LIGA technique have been developed at the Karolsruhe Nuclear Research center in Germany (see, e.g., Schomburg et al., Journal of Micromechanical Microengineering (1994) 4: 186-191), and commercialized by MicroParts (Dortmund, Germany). Jenoptik (Jena, Germany) also uses LIGA and a hot-embossing technique. Imprinting methods in PMMA have also been demonstrated (see, Martynova et.al., Analytical Chemistry (1997) 69: 4783-4789) However, these techniques do not lend themselves to rapid prototyping and manufacturing flexibility. Additionally, the foregoing references teach only the preparation of planar microfluidic structures. Moreover, the tool-up costs for both of these techniques are quite high and can be cost-prohibitive.
Various conventional tools and combinations of tools are used for separations and detections when performing analyses in conventional macroscopic volumes. Such tools include, for example: filters, metering devices, columns, valves, sample injectors, heaters, coolers, mixers, splitters, diverters, and electrodes (such as are used to induce electrokinetic flow and to perform electrophoretic separations). Attempts to conduct separations or detections in microfluidic volumes have been stifled by difficulties such as making such tools in microfluidic scale and then integrating such tools into microfluidic devices. Another difficulty is accurately measuring stoichiometric microfluidic volumes of reagents and solvents to perform analyses on a microfluidic scale. Additionally, difficulties in rapidly prototyping microfluidic devices are compounded by attempts to incorporate multiple analytical tools.
When working with fluids in conventional macroscopic volumes, achieving effective mixing between two or more fluid streams is a relatively straightforward task. Various conventional strategies may be employed to induce turbulent regions that cause fluid streams to mix rapidly. For example, active stirring or mixing elements (e.g., mechanically or magnetically driven) may be employed. Alternatively, special geometries may be employed in flow channels to promote mixing without the use of moving elements. One common example of the use of special geometries includes the addition of baffles to deflect flowing fluid streams and thereby promote turbulence.
Applying conventional mixing strategies to microfluidic volumes is generally ineffective, impractical, or both. To begin with, microfluidic systems are characterized by extremely high surface-to-volume ratios and correspondingly low Reynolds numbers (less than 2000) for most achievable fluid flow rates. At such low Reynolds numbers, fluid flow within most microfluidic systems is squarely within the laminar regime, and mixing between fluid streams is motivated primarily by the phenomenon of diffusion—typically a relatively slow process. In the laminar regime, using conventional geometric modifications such as baffles is generally ineffective for promoting mixing. Moreover, the task of integrating moveable stirring elements and/or their drive means in microfluidic devices would be prohibitively difficult using conventional means due to volumetric and/or cost constraints, in addition to concerns regarding their complexity and reliability. In light of these limitations, it would be desirable to provide a microfluidic mixer that could rapidly mix fluid streams without moving parts, in a minimal space, and at a very low construction cost. An ideal fluid mixer would further be characterized by minimal dead volume to facilitate mixing of extremely small fluid volumes.